Method and composition for mounting an electronic component and device formed therewith

ABSTRACT

A method of mounting a component on a substrate includes applying a conductive adhesive on a contact pad joined to a substrate, aligning a component with the substrate such that at least one lead of the component is juxtaposed with the conductive adhesive, performing a partial cure of the conductive adhesive, testing performance of the component, and performing a full cure of the conductive adhesive. Another method includes the additional steps of applying a tacky film to the substrate and juxtaposing the component with the tacky film. When the testing in either embodiment shows a defective or misaligned component, the component may be replaced or repositioned by cold separation of the at least one component lead from the partially cured conductive adhesive. Optionally, additional conductive adhesive may be applied, when needed, before replacement or repositioning of a component. A composition for mounting a component on a circuit board includes a conductive adhesive screened on a contact pad, the conductive adhesive being partially cured, wherein the composition allows cold separation of the component from the substrate and may be fully cured by heat exposure. An apparatus is also provided including a component, a substrate, and a means for temporarily mounting and electrically connecting the component on the substrate.

This application is a divisional of application Ser. No. 09/067,940filed on Apr. 28, 1998.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to the field of mounting an electronic componenton a substrate, particularly on circuit boards. More specifically theinvention relates to a method and composition for mounting an electroniccomponent on a substrate and the device formed by using such a methodand composition.

2. Background Art

An increasing need exists in the electronics and/or computer industry toprovide electronic packages of components and substrates, whereinmultiple components are joined to a single substrate, such as a circuitboard, particularly a printed circuit board (PCB). In fabricating suchpackages, the components and substrate are typically tested individuallyprior to assembly. However, once the package is fully assembled,defective devices may still be discovered in subsequent testing of theoverall package due to problems at the system level, that is, problemsonly apparent when certain components and the substrate operate togetherin their respective systems. Also, problems may become apparent due toimproper alignment or orientation of a component on the substrate. Ineither case, the most common remedy of the defect is rework, that is,removal and replacement of a component with a new component orrepositioning of the component.

The rework process is time consuming and can introduce new defects intothe package, other components, and/or the substrate. Often, solder pasteis used to attach components to a substrate. The soldered connectionsthat result from the soldering process are conductive and durable, asdesired, however, they are also difficult to rework. A componentattached with solder is generally removed by using a soldering iron orhot gas removal tools. The goal is to remove selectively and reattachonly the desired components without effecting the delicate componentsthat neighbor the desired components on the substrate. Nevertheless,whenever a soldering iron, hot gas tools, or other heated tools are usedit is extremely difficult to avoid exposing neighboring components tothe heat as well. The heat exposure may damage a component, thesubstrate, or a solder connection or otherwise introduce another defectinto the package. Accordingly, a removal process that does not requireheat is needed.

In some packages, adhesives are used to attach components to thesubstrate. The adhesives are often either conductive or nonconductiveepoxies, wherein conductive epoxies are used in a similar fashion tosolder paste. That is, the conductive epoxy is applied to the substrate,the electrical leads of a component are juxtaposed with the epoxy, andthen heat is usually required to cure the epoxy and create asufficiently strong mechanical bond and sufficiently reliable electricalconnection. When solder paste is used, a similar process is followedexcept that the heat causes the solder paste to flow into a molten stateand harden after removal of the heat. The ability to later reflow thesolder allows reworking of soldered components, however, existingtechnology does not provide a process for reworking cured conductiveepoxy. Accordingly, even if subsequent testing showed that an epoxiedpackage was defective, current practices would not allow for thereplacement or repositioning of selected components. For this reason,the inability to rework conductive epoxy is presently considered adisadvantage of such technology. Accordingly, an attachment process thatalso allows reworking is needed.

Attempts have been made to remedy the problems discussed above, however,they have met only limited success. Adhesive tape and conductive resinhave been used for components with peripheral leads, wherein the leadselectrically connect to contact pads on the substrate surrounding theperimeter of the component. The adhesive tape is placed between thecomponent and the substrate to provide a mechanical connection and theconductive resin is placed on the contact pads to provide an electricalconnection with the peripheral leads. The conductive resin does not bondthe leads to the pads, it simply provides a conduction path, thus, thecomponent may be replaced or repositioned by removing it from theadhesive tape. Accordingly, the removal process does not require heatand the attachment process allows reworking, but the electricalconnection has proven unreliable. Because the conductive resin does notprovide a mechanical bond of the leads to the contact pads, theelectrical connection may be easily compromised. The adhesive tape mightbe sufficient to bond the component to the substrate, but it is notsufficient to keep the leads electrically connected to the contact pads.Accordingly, an attachment process that provides a durable electricalconnection is also needed.

Notably, the tape and resin process described above is applicable to alimited number of component types. For example, it would be impracticalto use the process with a component having for its leads an array ofconductive bumps between the component and the substrate, providing bothan electrical and a mechanical connection. The difficulty of applyingadhesive tape amongst an array of contact pads on a substrate istypically sufficient to discourage use of the tape and resin process.Accordingly, an attachment process applicable to a variety of componentsis also needed.

Thus, it can be seen from the above discussion that it would be animprovement in the art to provide a method for forming durableconnections between a variety of components and substrates, wherein theconnections do not require heat for reworking. In other words, theability to rework is needed while providing durable and reliableconnections, wherein the rework has little impact on the substrate andneighboring components and connections.

DISCLOSURE OF INVENTION

According to the present invention, a method is provided for mounting acomponent on a substrate by: applying a conductive adhesive on a contactpad joined to a substrate; aligning a component with the substrate suchthat at least one lead of the component is juxtaposed with theconductive adhesive; performing a partial cure of the conductiveadhesive, such that an electrical and mechanical connection suitable fortesting is formed; testing performance of the component; and performinga full cure of the conductive adhesive, such that a permanent connectionis formed. The method may include additional steps, for example,applying a tacky film to the substrate and juxtaposing the componentwith the tacky film. Also, for example, when the testing shows adefective or misaligned component, the component may be adjusted in itsposition or replaced or repositioned by cold separation of at least onecomponent lead from the partially cured conductive adhesive. Further,additional conductive adhesive may be applied, when needed, beforereplacement or repositioning of a component.

A composition is also provided for mounting a component on a substrateincluding a conductive adhesive applied on a contact pad, the conductiveadhesive being partially cured, wherein the composition allows coldseparation of the component from the substrate and may selectively befully cured to permanently mount the component. By way of example, theconductive adhesive may also be partially cured and/or fully cured byheat exposure.

An apparatus is also provided including a component, a substrate, and ameans for temporarily mounting and electrically connecting the componenton the substrate. In one example, the temporary mounting means may alsobe a means for selectively mounting the component permanently on thesubstrate. A partially cured conductive adhesive is one example of botha temporary mounting means and a selectively permanent mounting means.Tacky film may also be used in combination with the conductive adhesive.

The foregoing and other features and advantages of the present inventionwill be apparent from the following more particular description ofpreferred embodiments of the invention, as illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

Preferred embodiments of the present invention will hereinafter bedescribed in conjunction with the appended drawings, where likedesignations denote like elements, and:

FIG. 1 is flow diagram showing a method according to the presentinvention;

FIG. 2 is a continuation of the flow diagram in FIG. 1;

FIG. 3 is a partial side view of an electronic package with a componenthaving array connections that are mounted to a circuit board accordingto a preferred embodiment of the present invention; and

FIG. 4 is a partial side view of an electronic package with a componenthaving peripheral leads that are mounted to a circuit board according toa preferred embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

According to a preferred embodiment of the present invention, a methodis provided for mounting a component on a substrate by applying aconductive adhesive on a contact pad joined to a substrate, aligning acomponent with the substrate such that at least one lead of thecomponent is juxtaposed with the conductive adhesive, performing apartial cure of the conductive adhesive, testing performance of thecomponent, and performing a full cure of the conductive adhesive. Inanother preferred embodiment, the method includes the additional stepsof applying a tacky film to the substrate and juxtaposing the componentwith the tacky film. Preferably, when the testing in either embodimentshows a defective or misaligned component, the component is replaced orrepositioned by cold separation of the at least one component lead fromthe partially cured conductive adhesive. Most preferably, additionalconductive adhesive is applied, when needed, before replacement orrepositioning of a component.

According to a preferred embodiment of the present invention, acomposition is also provided for mounting a component on a substrateincluding a conductive adhesive applied on a contact pad, the conductiveadhesive being partially cured, wherein the composition allows coldseparation of the component from the substrate and may selectively befully cured to permanently mount the component. Preferably, theconductive adhesive may also be partially cured and/or fully cured byheat exposure.

According to a preferred embodiment of the present invention, anapparatus is also provided including a component, a substrate, and ameans for temporarily mounting and electrically connecting the componenton the substrate. Preferably, the temporary mounting means may also be ameans for selectively mounting the component permanently on thesubstrate. Most preferably, a partially cured conductive adhesive is thetemporary mounting means and selectively permanent mounting means. Inanother preferred embodiment, tacky film is used in combination with theconductive adhesive.

Referring to FIGS. 1 and 2, the flow diagram displays a method 100 formounting a component on a substrate. Method 100 includes step 105 ofapplying a conductive adhesive on a contact pad joined to a substrate.Any method of applying conductive adhesive known to those skilled in theart is suitable, however, screen printing is preferred. Contact pads ofvarious configurations are typically provided on substrates for formingelectrical, and often mechanical, connections to components. Forexample, an array of contact pads may be provided for a component havingfor its leads an array of conductive bumps. Also, contact pads may beprovided to surround the perimeter of a component having peripheralleads. Other configurations of contact pads may also be provided. Thesubstrate may be of almost any type, but preferably is a circuit board,particularly a printed circuit board. Also, although step 105 and thediscussion below describe applying conductive adhesive on a contact pad,it is also understood that the adhesive may alternatively be applied tocomponent leads. The characteristics of a suitable conductive adhesiveand a few exemplary adhesives are described below.

Step 110 includes positioning a component on the substrate by aligning acomponent with the substrate such that at least one lead of thecomponent is juxtaposed with the conductive adhesive. The component maybe of almost any type, including those presently mounted on substrateswith the use of solder, adhesives, tape, etc. The leads of the componentare the conductive devices typically used to electrically connect thecomponent to the substrate. In the present invention, the leads alsomechanically connect the component to the substrate, in particular, tothe contact pads, although other devices may be provided that alsomechanically connect the component to the substrate.

For example, another preferred embodiment of method 100 includes anadditional step (not shown) of applying a tacky film to the substrateand juxtaposing the component with the tacky film. The tacky film couldalternatively be applied to the component. The tacky film increases thestrength of the mechanical connection, otherwise provided by theconductive adhesive applied in step 105, but should still allowreplacement or repositioning of the component when necessary. Any methodof applying tacky film known to those skilled in the art is suitable andwill depend upon the type of tacky film used. The tacky film ispreferably pressure sensitive tape, including tapes with silicon oracrylic-based adhesive. Some types of tacky film may also be preformed,wherein they are cut and/or stamped to have a predeterminedtwo-dimensional or three-dimensional shape so that they may be easilyapplied to the appropriate surface.

One example of a suitable acrylic-based adhesive tape is SCOTCH brand467MP or 468MP Roll Laminating Adhesive available from 3M Corp. inMinneapolis, Minn. Alternatively, the tacky film may be other materials,such as a layer of silicon or acrylic-based adhesive applied directly tothe substrate, for example by screen printing. A few examples of othersuitable adhesives are the partially curable nonconductive polymer resincoatings of U.S. Pat. No. 3,955,024 entitled “PRINTED CIRCUIT BOARD” toGoldman et al. issued on May 4, 1976 which is hereby incorporated byreference for its pertinent and supportive teachings. Thermosetmaterials may be used for the tacky film, wherein application of heatforms the mechanical connection between the component and the substrate.However, it should be remembered that the mechanical connection shouldbe temporary, allowing replacement or repositioning of the componentwhen necessary. Thermoplastic materials may also be used, whereinapplication of heat allows replacement or repositioning of thecomponent. Caution should be exercised with using thermoplasticmaterials, since the heat required for removal should be less than theheat required for removal of solder-connected components.

Generally, it is preferable that the tacky film be non-conductive toprevent electrical shorts between contact pads or between componentleads that the film might touch. However, if there is little possibilityof electrical shorts, then conductive tacky film could be used, eventhough not necessary. The purpose of the tacky film is simply to provideadditional strength to the mechanical connection between the componentand substrate. Application of the tacky film may be useful only forlimited types of components. For example, tacky film may be easilyapplied to form a mechanical connection between a substrate and acomponent with peripheral leads. However, it may be difficult to applytacky film to the substrate or component when array connections areused.

Next in FIG. 1, step 115 includes performing a partial cure of theapplied adhesive. Partially curing the conductive adhesive allows forreworking without heat, yet forms a temporary electrical and mechanicalconnection suitable for testing that may be selectively transformed intoa permanent connection. Preferably the conductive adhesive is partiallycured by heat exposure, although, it is conceivable that other curingmechanisms could be provided. For example, the partial cure could becompleted by radiation exposure, such as visible light illumination, orchemical exposure, such as catalysts or gases that partially cure theconductive adhesive. It is even conceivable that the applied adhesivepartially cures at ambient conditions without special processing andlater processing completes the full cure described below. Accordingly,the nature of any partial cure will depend primarily on the compositionof the conductive adhesive used since multiple compositions presentlyavailable or to be developed later may be suitable. Exemplarycompositions and their corresponding processing conditions for a partialcure are described below.

In step 120, the electronic package assembled from the component andsubstrate is performance tested. Testing may include any testingtechniques known to those skilled in the art or later developed. It islikely that individual components and circuit boards were tested formanufacturing defects soon after their fabrication before the start ofmethod 100. Accordingly, the basic objective of testing in step 120 isprimarily to detect system level problems and connection defects. Systemlevel problems are primarily problems only apparent when certaincomponents and the substrate operate together in their respectivesystems. Connection defects are primarily from misalignment orinadequate attachment of components with contact pads. Nevertheless,testing in step 120 may also be used to identify defects in individualcomponents or substrates incurred after fabrication, such as frommishandling, or other defects not identified in previous testing.

If the electronic package passes the performance testing of step 120,then step 125 is executed to perform a full cure of the conductiveadhesive. The full cure is similar to the partial cure in that itsprocess conditions depend primarily upon the composition used andexamples are described below. However, the objectives of the full cureare different. For example, one objective is to complete the formationof an electrical and mechanical connection between the component andsubstrate, rendering the connection permanent. That is, once theconductive adhesive is fully cured, the connection generally cannot bealtered except, perhaps, by the application of heat as with solderedcomponents.

If the electronic package does NOT pass the performance testing of step120, then rework processing preferably occurs, although a manufacturermay opt not to rework as shown in step 130. The option exists in method100 for no rework to occur when circumstances dictate that as thepreferred option. FIG. 2 displays the preferred steps 135 through 155 ofmethod 100 for a rework process. Alternatively, a rework process neednot include all of steps 135 through 155, but instead may only includeonly steps 135 and 145. In step 135 the component is removed or adjustedin position as allowed by virtue of the partial cure in step 115. Thisis possible, as described above, because partial curing forms atemporary electrical and mechanical connection having sufficientstrength and conductive properties. Specifically, the position of thecomponent on the substrate does not change when the substrate is movedduring testing, yet, the connection is conductive and the component maybe selectively removed or adjusted in position when needed. Inconventional processes, a soldering iron, hot gas tools, other heatedtools, or additional process steps are generally required to remove oradjust the component. However, according to the present invention, nosuch tools or additional steps are required, rather the removal oradjustment occurs simply by grasping the component and pulling it offthe substrate or adjusting its position, respectively.

Once the component is removed or adjusted in position, the remainingadhesive is checked in step 140 to ensure that it is adequate, sinceportions of the adhesive may be dislodged or compromised during theremoval or adjustment step 135. If the remaining adhesive is adequate,then a removed component may be replaced with a new component or theremoved component repositioned as shown in step 145. If the componentwas only adjusted in position and not fully removed, then step 145 isnot necessary. Next, method 100 resumes with performance testing in step120 of the reworked electronic package. Although not shown, analternative method is also conceivable wherein the method resumes afterstep 145 with full cure of the adhesive in step 125 without repeatingperformance testing in step 120.

If the remaining adhesive is not found adequate in step 140, thenadditional conductive adhesive may be locally applied as in step 150prior to replacement or repositioning in step 155. Any method ofreapplying conductive adhesive known to those skilled in the art issuitable, however, screen printing is preferred. When reapplication ofadhesive occurs in step 150, method 100 preferably resumes with step 115of performing a partial cure. Although not shown, alternative methodsare also conceivable wherein the method resumes with performance testingin step 120 or full cure of the adhesive in step 125, skipping thepartial cure or performance testing, respectively. Accordingly, thepresent invention provides a method for forming durable connectionsbetween a variety of components and substrates, wherein the connectionsdo not require heat for reworking.

Referring to FIG. 3, the side view displays part of an electronicpackage 300 including a component 310 and a substrate 320. Component 310has leads 330 arranged in an array and substrate 320 has contact pads340 arranged in a corresponding array to match up with array leads 330.Leads 330 are temporarily joined to corresponding contact pads 340 by apartially cured conductive adhesive 350. Partially cured conductiveadhesive 350 allows for reworking of package 300 without heat, yet formsan electrical and mechanical connection suitable for testing that may beselectively transformed into a permanent connection. That is, aconnection suitable to allow placing electronic package 300 in service.Exemplary components suitable for the preferred embodiment shown in FIG.3 include a ball-grid array (BGA), flip-chip, and other components witharray-type connections.

Referring to FIG. 4, the side view displays part of an electronicpackage 400 including a component 410 and a substrate 420. Component 410has peripheral leads 430 and substrate 420 has contact pads 440 arrangedin a corresponding pattern about the perimeter of component 410 to matchup with peripheral leads 430. Leads 430 are temporarily joined tocorresponding contact pads 440 by a partially cured conductive adhesive450. Partially cured conductive adhesive 450 allows for reworking ofpackage 400 without heat, yet forms an electrical and mechanicalconnection suitable for testing that may be selectively transformed intoa permanent connection. FIG. 4 also displays tacky film 460 positionedbetween component 410 and substrate 420 as described in method 100above. Exemplary components suitable for the preferred embodiment shownin FIG. 4 include a quad flat package (QFP), J-lead components,components suitable for tape automated bonding (TAB), and othercomponents with peripheral-type connections.

The present invention also provides a composition for mounting component310 or 410 on substrate 320 or 420, the composition including partiallycured conductive adhesive 350 or 450 applied on contact pad 340 or 440,wherein the composition allows rework as described in method 100 aboveand may selectively be fully cured to permanently mount component 310 or410. Preferably, conductive adhesive 350 or 450 is fully cured by heatexposure, and most preferably, it is also partially cured by heatexposure. Examples of specific compositions and curing processes aredescribed below.

EXAMPLE 1

EPO-TEK E2101 and EPO-TEK H20E-PFC (two-component silver filled epoxiesavailable from Epoxy Technology, Inc. in Billerica, Mass.) may be usedin the method, apparatus, and composition described above. Partialcuring of the epoxies is performed at a bond line temperature of 70° C.(Celsius)±5° C. for 30 min (minutes)±5 min. Full curing of E2101 isperformed at a bond line temperature of 175° C. for 15 min or 150° C.for 60 min. Full curing of H20E-PFC is performed at a minimum bond linetemperature of 175° C. for 45 sec (seconds), 150° C. for 5 min, 120° C.for 15 min, 80° C. for 90 min, or 50° C. for 12 hours. Full curing mayalso be otherwise according to the manufacturer's specifications.

EXAMPLE 2

EPO-TEK E3114-PFC and EPO-TEK E3116 (one-component silver filled epoxiesavailable from Epoxy Technology, Inc. in Billerica, Mass.) may be usedin the method, apparatus, and composition described above. Partialcuring of the epoxies is performed at a bond line temperature of 100°C.±5° C. for 20 min±5 min. Full curing of E3114-PFC is performed at aminimum bond line temperature of 200° C. for 30 sec, 175° C. for 45 sec,160° C. for 5 min, or 150° C. for 15 min. Full curing of E3116 isperformed at a minimum bond line temperature of 200° C. for 15 sec, 175°C. for 45 sec, 160° C. for 5 min, or 150° C. for 15 min. Full curing mayalso be otherwise according to the manufacturer's specifications.

EXAMPLE 3

The conductive adhesive of U.S. Pat. No. 5,372,750 entitled“ELECTRICALLY CONDUCTIVE SCREEN PRINTABLE COMPOSITION AND METHOD OFMAKING SAME” to Toma issued on Dec. 13, 1994 may also be partially curedaccording to the present invention.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention. Accordingly, unless otherwise specified, any dimensions orpositions of the apparatus indicated in the drawings or herein are givenas an example of possible dimensions and positions and not as alimitation. Similarly, unless otherwise specified, any sequence of stepsof the method indicated in the drawings or herein are given as anexample of a possible sequence and not as a limitation. For example, itwill be understood that, while various of the conductors (connections)are shown in the drawing as single lines, they are not so shown in alimiting sense, and may comprise plural conductor (connections), as isunderstood in the art.

What is claimed is:
 1. A system, comprising: a substrate having anelectrically conductive pad therein; an uncured electrically conductiveadhesive layer on the pad, wherein the adhesive layer includes anuncured electrically conductive adhesive, and wherein the adhesive layeris in mechanical and electrical contact with the pad; a componentcoupled to the substrate, wherein the component comprises a lead that isin mechanical and electrical contact with the adhesive layer so as toform an electrical connection between the component and the pad, andwherein the adhesive layer is not in mechanical contact with thecomponent; and partial curing means for partially curing the adhesivelayer throughout the adhesive layer while the adhesive layer is on thepad and is not in mechanical contact with the component, wherein thecomponent is coupled to the substrate by an interfacing electricallynon-conductive tacky film that is in mechanical contact with both thecomponent and the substrate.
 2. The system of claim 1, wherein the tackyfilm includes pressure sensitive tape.
 3. The system of claim 1, whereinthe adhesive includes pressure silver-filled epoxy.
 4. The system ofclaim 1, wherein the partial curing means includes heating means.
 5. Thesystem of claim 1, wherein the partial curing means includes radiationmeans.
 6. The system of claim 1, wherein the partial curing meansincludes chemical means.
 7. The system of claim 1, wherein the partialcuring means forms a temporary mechanical bond between the pad and thelead.
 8. The system of claim 1, wherein the substrate comprises acircuit board.
 9. The system of claim 1, wherein the components isselected from the group consisting of a ball-grid array (BGA) device, aflip-chip, components with array-type connections, a quad flat package(QFP), components suitable for tape automated bonding (TAB), andcomponents with peripheral-type connections.
 10. The system of claim 1,further comprising: testing means for testing the component anddetermining whether the component is defective or misaligned; replacingmeans for replacing the component, if defective, with a new componentand adjusting the position of or repositioning the component ifmisaligned; and full curing means for fully curing the adhesive layerthroughout the adhesive layer, said fully curing providing permanentfixed mechanical and electrical connections between the pad and thelead.
 11. A system comprising: a substrate having an electricallyconductive pad thereon; an partially cured electrically conductiveadhesive layer on the pad, wherein the adhesive layer includes apartially cured electrically conductive adhesive, and wherein theadhesive layer is in mechanical and electrical contact with the pad; anda component coupled to the substrate by an interfacing electricallynon-conductive tacky film that is in mechanical contact with both thecomponent and the substrate, wherein the component comprises a lead thatis in mechanical and electrical contact with the adhesive layer so as toform an electrical connection between the component and the pad, andwherein the adhesive layer is not in mechanical contact with thecomponent.
 12. The system of claim 11, wherein the tacky film includespressure sensitive tape.
 13. The system of claim 11, wherein theadhesive includes silver-filled epoxy.
 14. The system of claim 11,wherein a temporary mechanical bond exists between the pad and the lead.15. The system of claim 11, wherein the substrate comprises a circuitboard.
 16. The system of claim 11, wherein the component is selectedfrom the group consisting of a ball-grid array (BGA) device, aflip-chip, component with array-type connections, a quad flat package(QFP), components suitable for tape automated bonding (TAB), andcomponents with peripheral-type connections.